Surface relaxation of vapour-depositing colloidal glasses

TL;DR

This study reveals the microscopic relaxation mechanisms in vapour-deposited colloidal glasses, showing a two-step process involving surface out-of-cage motions and cooperative rearrangements that influence aging and stability.

Contribution

It provides the first direct observation of the microscopic kinetics of surface relaxation in vapour-deposited glasses at the single-particle level.

Findings

Surface particles relax via out-of-cage motions.

Deeper particles relax through cooperative rearrangement regions (CRRs).

CRRs are larger, more anisotropic, and more frequent near the surface.

Abstract

Vapour deposition can directly produce ultrastable glasses, which are similar to conventional glasses aged over thousands of years. The highly mobile surface layer is believed to accelerate the ageing process of vapour-deposited glasses, but its microscopic kinetics has not been experimentally observed. Here we studied the deposition growth kinetics of a two-dimensional colloidal glass at the single-particle level using video microscopy. We found that newly deposited particles in the surface layer (depth $d<14$ particles) relaxed via frequent out-of-cage motions, while particles in the deeper middle layer ($14<d\lesssim100$ particles) relaxed via activation of cooperative rearrangement regions (CRRs). These CRRs were much larger, more anisotropic and occurred more frequently than CRRs in the bulk ($d\gtrsim100$ particles) or after deposition. Their centers of mass moved towards the surface, while the particles within moved towards the bulk, causing free-volume "bubbles" to move towards the surface to give a more compact bulk glass. This two-step relaxation in two surface layers is distinct from the previously assumed relaxation in one surface mobile layer